I've looked for some information on this, but couldn't find anything useful. Has there been any noteworthy attempt to estimate the sum amount of individuals of all species that have ever lived on Earth?

2 Answers
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This is a hard problem - estimates of total living things in an given environment are usually created by looking at the number of species and individuals found in a sample area and extrapolating. As far as estimating the number of living things in the world, there are still lots of species which are not known, making this number still unknown for the world today as it is. Doing a similar survey into past ages adds another layer of problems to the estimate.

EO Wilson has, among others I'm sure, made a serious effort to understand how many species are currently alive on Earth. Its pretty clear that currently we don't know how many species are currently alive - that would make it nearly impossible to answer your question historically.

As you can see there are some important technical obstacles to even the question of how many species there are. Bacteria and free living eukaryotes have rules for speciation that are fairly difficult to define, esp since many can transfer just a few genes with many near relatives and do not require sexual reproduction.

Another obstacle is that the animal and plant surveys are not really done yet either. Plants animals and insects are still being discovered in jungles, deep underwater and at rest stops. As far as the microbial record goes, an average glass of water from a pond or river probably has undocumented species.

The encyclopedia of life is a resource worth mentioning here. It is self admittedly only a partial record.

For past speciation records, the geographical sampling is not strong - digging down in selected spots where fossil records remain is bound to miss many of the species which had existed.

The question asked for the number of individuals, which is different than the number of species. For reproduction by binary fission, would one (and which one--e.g., the longest survivor?) count as the original individual or are both new individuals? When does a colony or symbiotic collective become a single organism (e.g., mitochondria are presumably not considered distinct individuals but considering the biosphere a single organism seems to be going too far)? The high and highly variable rate of bacterial reproduction would make estimating this count problematic.
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Paul A. ClaytonJan 28 '13 at 15:02

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I tried to amend this... but you're right Paul I did misread this. I guess my eyes misled me because the question seems even more impossible. Estimates of bacteria living deep into the earth's crust have seriously multiplied in the past decade or two since they have been discovered and is still being discovered..newscientist.com/article/… Although the question of how many living animals would be more likely, they are hard since only select environments can be surveyed via fossil records. still many dinosaurs, etc to discover.
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shigetaJan 28 '13 at 18:49

it is impossible to know the exact number so here is my gross ballpark estimate of an upper bound - i.e. the maximum number of organisms that could have lived on earth in the extreme best case scenario. in practice it is probably much less, but this is to get an idea of what kind of numbers we are dealing with.

Bacteria take time until they reproduce. If they reproduce faster than they grow, they will become reduced to zero size, so there must be a balance between growth and reproduction.

Under ideal conditions the E-coli bacterium would divide every twenty minutes. source

Let's say we have the maximum number of bacteria on earth dividing every twenty minutes. Somehow, they divide and then one dies so that the population is stable. (hypothetically)

The age of the earth is roughly 4.5 Billion years. Much of that time, it was not colonised, but just to give an upper bound let's assume it was colonized the whole time.

There are 525600 minutes in a year so this give

4.5 ^ 10^9 * 5.3* 10^5 = 2.385 * 10^15 minutes

Dividing by twenty minutes gives 1.1925 * 10^14 total reproductions.

Hence, the number of organisms that ever lived if the entire volume of the crust of the earth were entirely filled with e-coli bacteria reproducing at the maximum of every 20 minutes would be:

(1.2*10^14) * 1.3*10^36 bacteria = 1.56 * 10^50 bacteria

This is an highly exaggerated gross estimate of the maximum number of organisms that ever lived in the extreme best case scenario. In practice it is probably 5-15 orders of magnitude less than this but we get an idea of the kind of numbers we are dealing with.

UPDATE: A more realistic number

The number of bacteria currently on earth is about 5*10^30 forming a biomass which exceeds that of all plants and animals. source.

Most of the earth’s prokaryotes (bacteria) occur in the open ocean, in soil, and
in oceanic and terrestrial subsurfaces, where the numbers of cells are
1.2 × 10^29, 2.6 × 10^29, 3.5 × 10^30, and 0.25–2.5 × 10^30, respectively. The numbers of heterotrophic prokaryotes in the upper
200 m of the open ocean, the ocean below 200 m, and soil are
consistent with average turnover times of 6–25 days, 0.8 yr, and 2.5
yr, respectively. Although subject to a great deal of uncertainty, the
estimate for the average turnover time of prokaryotes in the
subsurface is on the order of 1–2 × 10^3 yr. The cellular production
rate for all prokaryotes on earth is estimated at 1.7 × 10^30 cells/yr
and is highest in the open ocean.

Hence we have about 1.7 x 10^30 New Cells/year

Multiply this by the estimated 3.5 Billion years (3.5*10^9) when the earth was colonized and we get

5.95 * 10^39 Bacteria that ever lived. of course there were other organisms, but as above even the biomass of bacteria exceeds that of
all plants and animals combined so their effect on this number is
negligible. Likewise, the life conditions were surely not ideal at all
times, so this number may be lower still.

For some perspective, the estimated number of atoms comprising all of planet earth is about 1.33 * 10^50 according to this source.

ESTIMATE OF NUMBER OF MAMMALS THAT EVER EXISTED

The upper bound for mammals is as follows.

The surface area of the land of the earth is 149 million square kilometers.

Let us consider if the entire land area of the earth was completely covered with mice.

A mouse is typically 10cm from nose to base of tail and about 5 cm wide=0.1*.05=0.005 square meters. So we get 200 mice per square meter.

Hence, if the entire land area of the earth were covered with mice the world population of mice would be:

In practice mice need space and resources and compete with other mammals which take up space and which reproduce less frequently. Likewise, deserts and polar regions are less inhabitable, and sometimes there are periods of droughts, and they don't always reproduce at the maximum rate etc. so the total number is probably about around 4-10 orders of magnitude less than this.

Sorry, but this estimation makes no sense. First, the earth wasn't colonized from the very beginning, as far as I remember the oldest estimates for life go back as far as 3.5 billion years. Then life was different at that point (for example was not dependent on oxygen) and the life conditions were surely not ideal at all time. Not to mention the changes in atmosphere (and biosphere), when the oxygen concentration went up. It also assumes that all organims where from one kind of bacteria, which they were not.
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Chris♦Jul 10 '14 at 7:07

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@Chris I am trying to give an upper bound - i.e. a maximum number of organisms that could have lived on earth. this is the best we can do. I updated the answer with your point. primitive organisms were more or less the size of e-coli give or take an order of magnitude or two or three. do you think the upper bound number is higher? if yes, what would you change to increase this number?
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user813801Jul 10 '14 at 7:28

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20 min for generation time is way, way off. In natural conditions, like for example in soil, your average bacterium reproduces maybe once a week. I imagine generation times are still much longer in e.g. seawater.
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5heikkiJul 10 '14 at 16:06

@5heikki thank you for the clarification. yes, I wrote in the end, it is probably at least 5-15 orders of magnitude less. this is just an exaggerated upper bound best-case scenario. I updated it with a more realistic number. It's probably still less than this though.
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user813801Jul 11 '14 at 5:51

When looking at these numbers, did you happen to see anything similar about viruses?
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BrannonFeb 7 at 2:32